Phase Separation

Phase separation is what happens when an emulsion stops holding together and its oil and water parts drift back apart. In a bottle it shows as a clear layer over a cloudy one, a watery liquid released around a thickened mass, or a cream that has gone grainy and no longer spreads evenly.

Emulsions are held in a state they do not naturally want to be in. Emulsifiers, thickeners and stabilisers keep the droplets small and dispersed, and anything that disturbs that arrangement can undo it: heat in a car or a bathroom, freezing in transit, repeated temperature swings, contamination from fingers, or simply age.

Some products separate by design and say so — a two-layer remover or a shaken oil-and-water lotion is meant to be recombined before use.

• Unintended separation in a product that was previously uniform is a sign it has changed.
• A change in smell or colour alongside it is a stronger sign still.
• A separated emulsion cannot be reliably fixed by stirring, because the preservative and active distribution have changed too.

Storing products away from heat and direct sun, keeping caps closed and using clean hands or a spatula all slow this down. When a product has clearly separated, replacing it is the sensible course rather than testing it on skin.